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The Red Sirius
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[Author¡¯sNote: The following is a slightly abridged version of a chapter of abook in progress about the constellations and star-names of Greeceand Mesopotamia. The problem of the apparent color change in Siriusbetween ancient times and the present was first raised about 250years ago and has gotten worse instead of better because of thefindings and hypotheses of modern astrophysics. The observations ofthe Graeco-Roman authors quoted below have been long known; but theevidence from ancient Mesopotamia is seldom offered. The mostfamous¡ªand notorious¡ªdiscussion of the matter was by thecontroversial American astronomer T. J. J. See in the 1926 volume ofthe German periodical AstronomischeNachrichten.]
One of themost vexing problems of the history of astronomy is the fact that,though today Sirius is conspicuously blue-white in color, everyClassical writer that says anything at all about the matter describesthe star in no uncertain terms as red or ruddy. First, Ptolemy, atthe end of his star-catalogue in Books VII-VIII of the Almagest(written c 140 AD),describes Sirius as hypocirros,¡°fiery red.¡± This shocking statement might be dismissedas a copyist¡¯s error (after all, the earliest surviving text ofthe Almagest waswritten hundreds of years after the original), or even as aslip-of-the-pen by Ptolemy himself except for the fact that in thesame section of the AlmagestPtolemy describes five other stars as ¡°fiery red¡±: Arcturus, Aldebaran, Pollux, Antares, and Betelgeuse. One mightquibble with the inclusion of Pollux in this list¡ªit looks adeep yellow to me¡ªbut it certainly is not merely white, letalone bluish.
If Ptolemywas alone among ancient authors in calling Sirius red, we still couldsay that the great astronomer, or one of his copyists, had beencaught napping. Unfortunately other ancient writers on astronomy sawSirius in the same light as Ptolemy.
In hisdescription of Sirius (Phaenomena326-34), the Greek poet Aratos (early 3rdcentury BC) uses the term poikilos,for the star. This word is somewhat ambiguous, and has beenvariously translated as ¡°brightly colored¡± or¡°scintillating¡± or ¡°variegated.¡± To help usout, fortunately we have two ancient translations into Latin of thePhaenomena bycompetent literary men, the orator and statesman Cicero (106-43 BC),and Germanicus Caesar 16/15 BC ¨C 19 AD), Rome¡¯s greatestgeneral under Augustus. Cicero, in his translation of Phaenomena326-34, writes, rutilo cum lumine claretfervidus ille Canis, ¡°with ruddy lightfervidly glows that dog.¡± Germanicus, in a rather freerrendering of the passage, nevertheless uses the term rutilis,and says that the Dog¡¯s mouth (marked by Sirius) ¡°vomitsflame¡±.
In hisNatural Questions (I i6), the Stoic philosopher Seneca (4 BC ¨C 65 AD), another masterof Latin prose, compares the relative redness of three specificcelestial bodies, writing, [I]n coelo quoquenon unus appareat color rerum , sed acrior sit caniculae rubor,Martis remissior, Jovis nullus: ¡°[I]nthe heavens also there does not appear one color of things, but theredness of the Dog Star is deeper, that of Mars milder, that ofJupiter nothing at all . . .¡± Seneca is certainly right aboutthe color of Jupiter, which has no ruddiness whatsoever, and aboutthe color-contrast between Jupiter and Mars, so he sounds like areliable witness concerning the redness of Sirius at that time.
Of theancient astronomers besides Ptolemy, only Geminos of Rhodes (early1st century BC)says anything about Sirius¡¯ color. But his comments supportPtolemy. In his remarks criticizing the popular belief that the heatof the ¡°Dog Days¡± of summer is the result of thecombination of Sirius¡¯ light with that of the Sun, Geminosstates, ¡°For this star is of the same nature as all the otherstars. And whether the stars be fiery, or whether they be merelybright, they all have the same power.¡± Geminos uses the wordpyrina, ¡°fiery,¡±to describe Sirius, which is comparable to Ptolemy¡¯shypocirrus, ¡°fieryred.¡±
Thenatural philosophers Columella (4-70 AD) and Pliny (23-79 AD) alsodescribe Sirius¡¯ color as reddish in no uncertain terms. Inwriting about roses Columella compares their color to Tyrian purple,the rising Sun, Sirius, and Mars. (De CultuHortorum X 286.) Pliny¡¯s monumentalNatural History isvery long, and includes many astronomical facts, but he calls onlythree celestial objects ardensor igneus: the risingSun, Mars, and Sirius. (Nat. Hist. II 18 47.)
Indirectevidence concerning the ancient redness of Sirius comes from theRoman fertility festival of the Floralia, which was held in Mayduring the Dog Star¡¯s helical setting. According to the Romangrammarian Sextus Pompeius Festus (late 2ndcentury AD), one of the rites of that festival was the sacrifice ofruddy dogs to the Dog Star. The sacrifice of dogs to the Dog Star bythe early Greeks is mentioned by Apollonius Rhodius (295-215 BC) inhis Argonautica (II516-27), though he does not explicitly state that the sacrificialvictims were ruddy. In both cases the sacrifices were considerednecessary to avert the evil influences of what was considered a mostsinister star.
Mesopotamianastronomy also provides indirect evidence for the ancient redness ofSirius. The evidence is mostly in the Sumerian name for the star: Kak-si-s¨¢, the¡°Arrow.¡± At the time the Sumerians gave ¦Á CanisMajoris this name, around 3000 BC, stone arrowheads had just beensuperseded by ruddy-colored copperarrowheads. Alternatively, a ruddy ¦Á CMa might have suggestedthe blood of an enemy on an arrowhead. However, the Sumerianconstellation of the ¡°Bow¡± in southern Canis Major andnorthern Puppis (¦Ê-¦Å-¦Ò-¦Ä-¦Ó-¦Ç CMaplus ¦Î-¦Ñ Puppis) is so well-defined and conspicuous, and ¦ÁCMa is oriented so suggestively with respect to it, that the starprobably would have ended up being named ¡°The Arrow¡±whatever its color.
As aweapon, the Sumerian celestial Arrow naturally was associated withthe old Sumerian war-god Ninurta. From the Royal Library ofAshurbanipal (668-626 BC) in the ruins of his palace at Nineveh, hassurvived a text that calls itself a ¡°Prayer to Kak-si-s¨¢when it stands at sunrise¡±¡ªin other words, as itheliacally rose. In its opening line it addresses ¡°Ninurta,mighty one, heroic god, prince of the Anunnaki, commander of theIgigi.¡± (The ¡°Anunnaki¡± and ¡°Igigi¡±were groups of gods; in fact ¡°Igigi¡± often referred tothe stars and constellations.) In this prayer Ninurta is called a¡°consuming fire, burning the wicked . . . Whose title in heavenis Kak-si-s¨¢;among the host of the Igigi mighty is thy [tablet broken]¡±. Itwill be remembered that Ptolemy and Geminos both described theruddiness of Sirius as ¡°fiery.¡± The association of a redstar with the god of war is natural, even inevitable. Antares andMars were also associated with war in both Greece and Mesopotamia.
Thus theevidence is overwhelming that Sirius looked red from the beginning ofrecorded history down at least to the end of Classical antiquity (4thcentury AD. Medieval Arabian texts say nothing about the star beingruddy.) The problem is how the star could have changed from ruddy toblue-white in little more than a millennium and a half¡ªfar tooshort a time for most astrophysical processes to have effected such aradical change. (Faster processes usually end up with a demolishedstar.) Sirius is an A1 main sequence object and as such went throughan early evolutionary phase during which it was a ruddy pre-mainsequence star¡ªbut that, according to theory, must have beentens of millions of years ago. An early suggestion was that perhapsduring ancient times Sirius was passing through a thick cloud ofinterstellar dust that reddened it. But to have reddened the star somuch, such a cloud would have seriously dimmedit as well, and Ptolemy¡¯s star-catalogue makes it clear thatSirius was every bit as bright thenas it is now. And inany case radio astronomy has detected no heavy dust in theneighborhood of Sirius. Indeed, the direction of Sirius is so poorin interstellar dust that the open cluster M41, located just 4¡ãto its south, is dimmed only 0.2 magnitude by dust even though it is2300 light-years away.
The besthope for an explanation of Sirius¡¯ dramatic change in colorfrom ancient times to the present is from its famous white dwarfcompanion. Here is a star that in the astrophysically-recent pastwas a red giant¡ªbut ¡°astrophysically-recent past¡±means a whole lot more than 1500 years ago! Moreover, in evolvingfrom a red giant to a white dwarf a star ejects most of the matter inits envelope (usually in the form of a planetary nebula), and in thecase of Sirius that material would still be in the star¡¯svicinity. But no such residual of matter ejection has been detectedaround Sirius. Furthermore, had Sirius B been a red giant 2000 yearsago, the Dog Star would have been several times brighter than theancients reported it to be.
The sheernumber of ancient authorities who testify to the ruddiness of Siriusduring their time deny us the luxury of writing this puzzlement offto ¡°copyist error.¡± And to say that people of theintellectual caliber of Ptolemy, Geminos, Seneca, Pliny, Columella,Cicero, and Germanicus, writers whose statements on other matters areconsistently accurate and reliable, could allhave been deceived about the color of the brightest fixed star in thesky would be absurd. The only alternative is that there is somethingwrong with our astrophysics. But is that so very improbable? Whowould claim that modern astrophysics, whatever its successes, iserror-free? Our understanding of the universe in general, and of thestars in particular, is still very tentative. We must always keepour hearts and our minds open. Only by considering ALL thepossibilities when confronted by a problem like that of the color ofSirius will we continue to make intellectual and scientific progress.